Electroless ni-fe deposition process



May 7, 1968 J. D. OLSEN ET 3,382,079

ELECTROLESS Ni F DEPOSITION PROCESS Filed Dec. 16, 1964 e E 22 p 322%; ge E 1 I 12 14 i i INVENTORS JUDITH D OLSEN LUBOMYR T. ROMANKIW ATTORNEYUnited States Patent ELECTROLESS Ni-Fe DEPOSITION PROCESS Judith D.Olsen, Ossining, and Lubomyr T. Romankiw,

Lake Mohegan, N.Y., assignors to International Business MachinesCorporation, Armonk, N.Y., a corporation of New York Filed Dec. 16,1964, Ser. No. 418,706 5 Claims. (Cl. 106--1) ABSTRACT OF THE DISCLOSUREAn iron or iron-nickel electroless deposition baths useful life isprolonged by preventing the oxidation thereof. The bath is purged by afinely dispersed stream of nitrogen gas bubbles, prior and subsequent tothe addition of ferrous ions to the bath. The nitrogen gas, prior toentering the electroless bath, is passed through a solution whose pH andtemperature are the same as that of the bath. Both the solution and bathcontain a common volatile base, so that the volatilized base of the bathis continuously replenished therein. The purging nitrogen gas alsoagitates the bath and results in a film free of oxide occlusions.

This invention relates to a process in which iron or nickel-iron alloyscan be electrolessly deposited onto a suitable conducting substrate, andmore particularly to a process for increasing the useful life of thebath in which said electroless deposition takes place.

The term electroless deposition as known in the art describes a processwherein a conducting substrate is inserted in a bath containing, inaqueous solution, a salt, such as a chloride or sulfate, of a metal suchas iron, nickel, cobalt and chromium, a reducing agent, such as sodiumhypophosphite, a suitable complexing agent such as sodium citrate orsodium potassium tartrate and a buffering agent where needed. In theensuing reaction, the iron, nickel, cobalt or chromium is deposited as apure metal on the surface of the substrate. Other baths permit alloys ofiron and nickel to be plated onto a suitable substrate. The art ofelectroless plating has been given extensive treatment in a publicationentitled, Symposium on Electroless Nickel Plating (Catalytic Depositionof Nickel-Phosphorus Alloys by Chemical Reduction in Aqueous Solution),identified as ASTM Special Technical Publication No. 265 and publishedby the American Society for Testing Materials in Philadelphia, Pa., in1959.

Electroless iron and electroless iron-nickel alloys are plated from analkaline bath. Iron and nickel in these baths are kept fromprecipitating as hydroxides by complexing the metallic ions withsuitable complexing agents. When all of the factors in the bath are keptconstant, the percent of iron in the alloy, as well as the bath life,depends on the ratio Fe(II)/Ni(ll) in the bath. Since Fe(II) oxidizesrapidly to Fe(III) in alkaline solutions, it becomes difficult tocontrol the rate of deposition and percent of Fe that will be plated ona substrate held within such bath. It is also noted that (H PO becomesoxidized and, as its concentration is changed, the rate of deposition ofNi-Fe changes.

The present invention prevents or considerably slows down the oxidationmentioned above by purging the bath, prior to the addition of theferrous salt to the bath as Well as throughout the life of the bath,with a continuous, finely dispersed stream of nitrogen gas bubbles. Thenitrogen gas, prior to entering the electroless bath, is passed througha solution whose pH and temperature are the same as that of the bath.Where, for example, the bath contains ammonia, the nitrogen gas isbubbled through an ammoniacal water solution Whose pH and temperature,the latter being the main controllers of the activity of the volatileconstituents, which in the instant case are NH and H 0, are nearly thesame as that of the bath. In short, where a bath contains a pHcontroller that is volatile, the purging nitrogen passes through asolution containing such volatile constituent.

While the theory of operation of the electroless process is not alwaysclearly understood, it has been observed that the life of the bath usedin an electroless deposition where iron deposition is involved isshortened due to a high oxidation of the iron and a bath having a highpH. Since iron and nickel-iron alloys are electrolessly deposited in analkaline source, ergo, a high pH, there is a strong tendency for theoxidation of the iron and (H PO F. It is this oxidation which must beavoided in order to extend the life of such bath. By passing nitrogenthrough a chamber containing ammonium hydroxide, finely dispersednitrogen will entrain the ammonia and carry it to the plating bath whereits presence compensates for loss of the pH controlling factor, namely,the ammonium hydroxide in the bath. The purging nitrogen also agitatesthe bath solution and results in a film free of oxide occlusions.

It is an object of this invention to provide a new and improved processfor the electroless deposition of iron and iron-nickel alloys.

It is yet another object to provide a new and improved process forextending the useful life of a bath employed in the electroless iron andiron-nickel alloy deposition.

It is a further object to improve electroless deposition processes bystabilizing the bath composition employed in such processes.

It is yet another object to attain uniform composition through thethickness of the deposited film.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention as illustrated inthe accompanying drawing.

The sole figure illustrates a means for carrying out the objects setforth hereinabove. As seen in the sole figure a container 2 houses asuitable plating solution 4, generally alkaline, and element 6 is asupport for holding an item 8 to be placed within the solution 4. Acover 10 comprises a layer of inert, organic material, such as siliconoil or xylene, the latter serving to slow down the oxidation rate of thebath. An additional cover 12 serves to cut down the evaporation rate ofthe plating solution and an opening 14 is provided in such cover for theescape of gases to relieve pressure of expansion.

Nitrogen gas N or other inert gas, is fed through tube 16 into asolution of NH OH and the nitrogen is dispersed into a fine stream ofgas bubbles by passing such nitrogen gas through fritted glass 20 beforeentering the ammonia solution 18. The ammoniated nitrogen passes throughvalve 22 and enters the plating solution after passing through anotherfritted glass exit port 24.

Representative plating solutions are composed of:

NiCl -6H 0 g./l 14 Fe(Nl-I (CO -6H O g./l 10.3 KnaC H O g./1 NaH PO2'H20g./l.... 6 NH OH ml./l 230 Nickel chloride g./1 14.0 Ferrous ammoniumsulfate g./l 6.5 Rochelle salt g./l 32.5 Sodium hypophosphite g./l.. 4.4Ammonium hydroxide m./l 230 The NH OH chosen for solution 18 wasmaintained at 55-75 C. so that a pH of 11.5 could be maintained. Whenanother plating solution composed of was used, 28 ml./l. of NH OH wasused to make the pH 8.2 at the plating temperature of the bath. Thenickel chloride serves as the source of nickel and the ferrous ammoniumsulfate is the source of iron and also serves as a buffer. The tartrateor Rochelle salt is the complexing agent and the sodium hypophosphite isthe reducing agent. The ammonium hydroxide is the controller ordeterminant of the pH of the bath solution and also serves to complexnickel.

The nitrogen entering at 16 exits from fritted member 20 as finelydispersed bubbles, the latter entraining the ammonia from solution 18and such ammoniated nitrogen, whose rate of entry into the bath isregulated by valve 22, is further dispersed by fritted member 24 beforeentering the bath solution 4. The finely dispersed bubbles not onlyagitate the bath at the deposition surface of sample 8, but also sweepsout any oxidating agent in the bath 4 and forms a protective atmosphericlayer under cover 12. Of course, the ammonia added to bath 4, replacesthe highly volatile ammonia that escapes through port 14, suchreplacement being necessary to maintain the pH of the bath solution 4.Other known alkaline baths, employed in the electroless deposition ofiron and nickel-iron, can have their lives extended so long as thenitrogen or other inert gas is employed to entrain that volatilesubstance in solution 18 which is employed in bath 4 to preserve the pHof the bath solution.

Although the purging feature is a primary Way of extending the life ofelectroless Fe and electroless Ni-Fe baths and to provide uniformcomposition free of oxide occlusions in the films, certain otherpractical considerations will help to extend the life of such baths. Itis recommended that the ratio of bath volume 4 to exposed surface areaof the bath be high. Moreover, the cover 12 should have a tight seal sothat the space above the xylene or silicon oil cover is filled primarilywith nitrogen and ammonia vapor in addition to the water vapor andhydrogen gas being produced during the chemical reactions taking placewithin the bath 4 so as to further avoid the presence of oxygen.

While the invention has been shown and described with reference to apreferred embodiment thereof, it will be understood by those skilled inthe art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention.

What is claimed is:

1. A method of extending the life of an electroless bath containingmetal ions selected from iron and ironnickel ions, hypophosphitereducing agents and a volatile base comprising the steps of passing aninert gas through an ammonium solution whose activity of its volatileconstituents is the same as said electroless bath, and then passing saidgas through said bath.

2. A method according to claim 1 wherein said bath contains iron ionsand said inert gas is passed through an ammonium solution whose pH andtemperature are the same as said electroless bath, and then passing saidgas through said bath.

3. A method according to claim 1 wherein said inert gas is nitrogen.

4. A method according to claim 1 wherein said volatile base is ammoniumhydroxide.

5. A method of extending the life of an electroless alkaline bathcomprising iron ions, hypophosphite reducing agent and a volatile pHcontroller comprising the steps of passing nitrogen gas through anammonium solution whose pH is the same as said electroless bath andwhich contains the pH controller of said bath, and then passing saidnitrogen through said bath.

References Cited UNITED STATES PATENTS 2,819,188 1/1958 Metheny et a1.117130 2,938,805 5/1960 Agens 117l30 X 3,261,711 7/1966 Sallo 117l30 X3,281,266 1/1966 Colonel 1l7--130 RALPH s. KENDALL, Primary Examiner.

